The invention relates generally to multi-layer coatings for substrates and more specifically to a multi-layer thin film on glass having good visible light transmittance and low emissivity. The glass product finds particular application in residential and commercial markets.
The emphasis upon expanses of glass in modern architecture has generated a commensurate interest in adjusting the optical performance of the glass to suit widely varying requirements. Not only the visibly perceived color but also the reflectance nd transmittance of the glass across the solar energy spectrum have become objects of examination and control. Some of the means whereby such performance parameters are controlled is the application of a broad selection of materials, typically metals, by various processes such as pyrolisis, chemical vapor deposition, vacuum evaporation and cathode disintegration, i.e., sputtering. Because glass lights having thin film coatings intended for architectural use are satisfactory only when the applied coating is uniform in both composition and thickness, sputtering is the process presently favored because of its inherently uniform application characteristics.
The sputtering of transparent metallic thin films on glass substrates is not new. For example, U.S. Pat. No. 2,047,351 teaches a two step sputtering process whereby a first 100 Angstrom layer of a hard, adherent metal such as iron, cobalt, nickel, or chromium is covered by a second sputtered layer of a precious metal such as platinum, palladium, gold or silver.
U.S. Pat. No. 2,825,687 discloses a method of sputtering a multiple layer film onto a glass substrate. The first layer of material is typically an oxide of a metal such as bismuth or antimony having a thickness in the range of from 100 to 600 Angstroms and a second, outer layer of gold having a thickness of from 75 to 280 Angstroms. It is there noted that the films disclosed have the practical advantage that they transmit or absorb only a small proportion of the infra-red radiation incident thereon and reflect the greater part of such radiation.
U.S. Pat. No. 3,400,006 teaches another solar energy controlling vacuum vapor deposited alloy coating. The coating is a gold based alloy with small amounts of germanium and chromium which is deposited on plate glass to a thickness in the range of about 125 Angstroms to 225 Angstroms. Depending upon thickness, this coating provides visible light transmittance of about 40% and total solar radiation transmittance in the range of about 10 to 30%.
U.S. Pat. Nos. 3,410,710 and 3,457,106 disclose other alloy films applied by chemical means which provide control of the transmittance and reflectance of visible and infra-red light. The coatings disclosed therein are typically from about 900 Angstroms thick to several thousand Angstroms thick.
U.S. Pat. No. 4,022,947 discloses an alloy film consisting of predominantly iron, nickel and chromium coated with an oxide film of substantially the same constituents. The films are sputter coated onto a glass substrate and provide a product having a transmittance at a wavelength of 500 nanometers of about 10% to 20% and a reflectance at 3,000 nanometers of between about 40% and 65%.
The foregoing patents are an abbreviated, but representative, selection of prior art teachings relating to the sputter coating of metal and metal alloy films on glass. While the basic product and process has changed little, the performance goals and the choice of materials to achieve such specific performance goals have. Initially, much emphasis was placed on the electrical performance of the product to achieve self-deicing translucent products for use in automobile and airplane windows. Emphasis then shifted toward control of infra-red invisible light transmission for windows intended for architectural service. Such products typically provide low visible light transmission, on the order of 10 to 20%, and provide greatly reduced infra-red energy transmission. Clearly such performance parameters were intended to and do provide greatly reduced solar load and thus commensurate reductions in summer air conditioning costs.
A contemporary development in such products relates to low emissivity or low E glass. While providing significant visible light transmission, typically about 80%, such products are intended to control the escape of energy re-radiated from low temperature objects within a building. That is, while providing good visible light transmission and reduced infra-red transmission, such products also provide enhanced reflection in the far infra-red region.